TecSAR (also spelling of TechSAR) is the first spaceborne radar minisatellite technology demonstration mission of Israel's MoD (Ministry of Defense), designed and developed by IAI/MBT (Israel Aerospace Industries Ltd.) formerly known as Israel Aircraft Industries Ltd., a government owned company. The prime objective is to provide high-resolution SAR imagery, day and night, in all weather conditions, at an affordable cost. 1)2)3)4)

Note: The military nature of the mission does not allow to provide any more specifics to the description of this mission. This applies in particular to such items as “swath width.” Some information on spatial resolutions is provided (Table 1).

Spacecraft:

The spacecraft features a low-mass design and a configuration (shape of bus with low moments of inertia about the roll axis), to provide a maximum of pointing agility. Combining high maneuverability with electronic antenna beam steering, TecSAR offers high-resolution coverage of large areas in various observation modes, among them also the mosaic form (event monitoring capability). 5)

The spacecraft is 3-axis stabilized, using a newly developed bus, with a launch mass of < 300 kg (260 kg dry mass, including a payload mass of 100 kg). The design keeps the bus and payload well separated so that changes due to growth potential in either one element will have a minimum effect on the other element. Electrical power of 750 W (EOL) is provided by two solar panels. The AOCS (Attitude and Orbit Control Subsystem) provides a high degree of pointing accuracy. An onboard data storage capacity of 240 Gbit is provided. The spacecraft design life is 5 years (minimum).

The highly agile bus design in combination with the body-pointing parabolic antenna dish system permits greatly increased viewing capabilities from the spacecraft. The spacecraft/antenna system may be dynamically redirected into any direction of the flight path (i.e. in the cross-track as well as in the along-track direction). Thus, a wide FOR (Field of Regard) within the incidence-angle range may be obtained on either side of the ground track for event monitoring coverage.

Note: In retrospect, the TecSAR spacecraft bus is referred to as OptSat-2000 platform (Ref. 8).

Figure 1: Illustration of the TecSAR spacecraft (image credit: IAI)

Launch: The launch of TecSAR took place on January 21, 2008 on a PSLV launcher of ISRO (Indian Space Research Organization) from SDSC (Satish Dhawan Space Center) at Sriharikota, India. IAI signed a launch contract with Antrix Corporation in Nov. 2005. 6)7)

Note: Despite the advantages of the Indian carrier over a Shavit rocket, the policy of the Ministry of Defence - preservation of Israel's independent launch capability - remains unchanged. 8)

RF communications: High-rate X-band downlink. The system is operated with a number of ground stations.

Mission status:

• The TecSAR spacecraft and its payload are operating nominally in the summer of 2014. Since the start of the operational phase, The TecSAR mission provides a few thousand images per month to the customer. 9)

- Image quality checks are performed on a regular basis several times a year. Since launch, no degradation in image quality was observed, the ISLR and PSLR figures remain stable.

- As of 2014, a second generation satellite, TecSAR-2, is under development. This satellite is characterized by a larger antenna, longer imaging ranges and multi-polarization capabilities. Due to the modular design of the TecSAR spacecraft, the implementation of these improvements does not require major changes to the hardware design.

• The TecSAR spacecraft and its payload are operating nominally in 2012. TecSAR has been producing thousands of high quality images per month since its launch in January 2008, to the users' full satisfaction. In Feb. 2012, IAI (Israel Aerospace Industries), the developer of TecSAR, won the Technological Innovation award at the "Technologies" group annual International Aerospace Conference and Exhibition in Jerusalem. 10)

• The TecSAR spacecraft and its payload are operating nominally in 2010.

• In June 2009, the Trinidad program (see below) hadn't received any funding so far from DoD due to budgetary problems. However, a US-Israeli team demonstrated the imaging capability of the TecSAR spacecraft in performance tests in Florida (3 weeks in June 2009). SAR imagery was downlinked, delivered and processed twice a day in a matter of 15 minutes for analysis by a user team. The demonstration validated to the military management the very responsive operational capability of the system. 11)

• After the IOT was successfully completed, the system was handed over to the customer for the operational phase.

- A quality check performed on the images reveals the excellent characteristics of the images. Indeed, the ISLR (Integrated Side Lobe Ratio) level is around 18 dB, while the PSLR (Peak Side Lobe Ratio) is about -30 dB. These measurements were performed during the IOT.

- Also, some repeat pass imagery was taken during IOT over several sites in Israel to test the interferometric capabilities of the system. The system's high spatial resolution leads to an enhanced interferometric performance with potentially fine DEM production capabilities.

• The first SAR imagery of TecSAR was received 10 days after launch in January 2008 (in the IOT phase).

Future uses of the TecSAR system: As of April 2007, Northrop Grumman Space Technology (NGST) of Redondo Beach, CA is partnering with Israel Aerospace Industries to adopt IAI's TecSAR satellite for use in future versions of ”operationally responsive space” applications of the US DoD. For this purpose, NGST will modify the satellite with additional payloads and specialized software. The upgraded satellite will be called ”Trinidad” and be available 28 months after contract signature.

NGST will combine its proven intelligence, surveillance and reconnaissance (ISR) mission systems engineering and systems integration capability with IAI's TecSAR high-resolution, synthetic aperture radar imaging satellite. Mission assurance capabilities, including secure communications and other US system requirements, will be incorporated into the spacecraft by NGST. Based on IAI's TecSAR multi-mode X-band radar imaging satellite, this new system can provide significant, near-term, day/night and all-weather point and area collection capability to meet the immediate needs of warfighters in theater as well as those of the broader intelligence community. 12)13)14)15)

The satellite will be operated with a compact, portable ground system that provides the flexibility to perform tasking and data dissemination from the continental USA or from any operational theater. The satellites can be individually launched from a low-cost Minotaur or Falcon 1 rocket, or as a group of four or more on an EELV-class launcher.

• The IOT (In Orbit Test) period started immediately after the spacecraft was launched.

Sensor complement: (XSAR)

XSAR (X-band Synthetic Aperture Radar):

XSAR was designed and developed by Elta Systems Ltd. of Ashdod, Israel, a subsidiary of IAI. The XSAR system has an airborne predecessor, a fully polarimetric X-band multimode instrument also of Elta Systems design and manufacture. Elta's multimode radar was flown on an airborne platform, a Boeing 737 (timeframe 2004), and data were provided at a resolution of 1m/ pixel. This airborne sensor served as a testbed for the XSAR payload of the TECSAR satellite. 16)

XSAR operates at a center frequency of 9.59 GHz (3.1 cm wavelength) with a revisit period of 3 or 4 days and a repeat cycle of 14 days, the look angle can vary from 20-45º and the instrument is capable of acquiring data from both left and right look directions of the subsatellite track.

The payload modules are separated from the bus so that only cables and wires connect the two. The payload modules occupy a section close to the antenna (Figures 5 and 9). The OBR and DLTU components are part of the bus modules.

The MTT is composed of 10 RF TWT amplifiers referred to as CTWTA (Channeled Traveling Wave Tube Amplifier). Only 8 CTWTAs (out of 10) are being used during an imaging phase; the other two are kept as cold redundancy. The MTT arrangement supports graceful degradation even if only seven of the ten CTWTAs remain in working order. The reflected signal passes through the feeder to a Front End (FE) that contains circulator and LNA (Low Noise Amplifier). A fast electronic switch at the receiver input selects the power coming from one out of the eight FEs. This power is amplified and sampled by a fast A/D (Analog/Digital) converter. The sampled data passes through a 6 to 3 bit per sample BFPQ (Block Floating Point Quantization) compression algorithm. The source data is then recorded onto the OBR (Onboard Recorder).

In the TecSAR system design, a derivative of the BFPQ algorithm is used, namely MBFPQ (Modified Block Floating Point Quantization). The MBFPQ algorithm exploits the fast CPU computing power of the payload (80486DX2 CPU) thus avoiding the need to assume a Gaussian distribution of the data. 19)

All electronics and RF devices (MTT and RSC) of the XSAR payload are part of the hexagonal container as shown in Figure 9.

The radiating element of XSAR is the parabolic antenna, a deployable umbrella reflector, with a rigid CFRP (Carbon Fiber Reinforced Plastic) central dish and a set of knitted mesh gores stretched by skeleton ribs. The ribs lie on the surface of a parent paraboloid where each gore surface has a paraboloid cylinder contour. The configuration of the antenna is shown in Figures 5 and 7. A high paraboloid reflector surface accuracy is achieved by ribs position adjustment after mesh gores mounting. The antenna measurement technique was verified using the method of stereo photogrammetry. The entire reflector mesh has a mass of < 0.5 kg.

High peak power dictates the overall mission power requirements. TecSAR needs 1.6 kW of power during imaging and the bus is designed to give 750 W (EOL).

The electronic beam steering capability is achieved by using an antenna feed array in the focal plane (the power generated in the MTT is being directed toward different feeders in the antenna feed array). Each feeder has a different position relative to the antenna focal point: hence, a multibeam pattern is created (Figure 6). The antenna includes a TTD (True Time Delay) function for the electronic steering with a very wideband beam. As a consequence of the wideband antenna and the TTD feature, the system's range resolution is not limited as in the case of an active phased array antenna.

All electronic circuits and RF modules are of the payload are installed into a hexagonal container, including the MTT and the RSC (Figure 9). The OBR and DLTU are mounted in the bus section of the spacecraft.

The multi-mode payload employs electronic beam steering, which can be operated in various observation modes including various polarization combinations (optional): HH, HV, VH, VV.

• Stripmap mode: the synthetic apertures are targeted on wide geographical swaths. The spacecraft performs synchronous imaging and does not change its orientation during observations except for some small maneuver due to the need to keep the imaging strip parallel to the ground track. Squinted strip imaging is possible.

• Wide coverage ScanSAR. The coverage of large strips is achieved by electronic beam steering. Three beams are used in the nominal wide coverage mode which create three footprints (subswaths) in the target area. The ground resolution in this mode is decreasing since the integration time is split up among the subswaths.
The swath width can be increased by using more antenna beams. In principle the swath width may get to more than 100 km for some incidence angles. However, this reduces the ground resolution to about 20 m.

• Spotlight mode: this focuses on a specific, pre-assigned target. In spotlight, the spacecraft performs mechanical steering to halt the antenna footprint in a specific target area. The longer integration time over the spot target area yields an improved azimuth resolution. The range resolution is achieved in adjusting the bandwidth to the incidence angle. The TecSAR ability for spotlight imaging in squint allows for multi-look imaging without any loss in resolution. To obtain a multi-look image of a given target area, a number of spotlight images are being observed, each at a different squint angle.

• Mosaic mode: the radar imager slews its focus on a number of spots in the same general target area. The mosaic mode enables to extend the limited coverage of the spot mode by using the electronic steering capability of XSAR. In mosaic mode the radar beam scans in the range direction while the mechanical maneuvering advances the strip line in the azimuth direction. Hence, this mode may also be interpreted as the spot version of ScanSAR.

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).